The present invention relates to the field of electron tubes, and especially cathodes whose function in these tubes is to emit electrons and thus constitute the source of an electron current.
More particularly, the invention relates to so-called oxide cathodes. These cathodes, which are the most widely used, comprise a layer of oxides which are strong electron emitters on one face of a metal support. The support is connected to an electric potential which is negative relative to the surrounding potential, allowing an electron flux to be emitted from the oxide layer.
FIG. 1 is a simplified sectional view showing a cross section through a conventional oxide cathode 2. The support 1 consists of a thin nickel plate forming a pill which has a face 1a covered with an oxide layer 3 in the form of a washcoat. The washcoat is a coating consisting of an active compound filler and of a binder. The active compound is generally based on barium carbonate (BaCO3) and on carbonates of other elements, which are subsequently converted to barium oxide (BaO) and oxides of other elements.
The oxide layer normally has to be at a relatively high temperature to emit. In the conventional case of a so-called indirectly-heated cathode, a heat source, such as a filament, is provided near the support and connected to a low-voltage current source.
In operation, an electron current flows though the thickness of the oxide layer 3 (arrow I) due to the effect of the surrounding electric field. The electric field is created by establishing a potential difference between the support 1 and an electrode 5 located near the external surface 3a of the layer 3. In the example, the support is referenced at an earth voltage while the electrode 5 is biased at a high positive voltage +V. The electron flux obtained by the cathode 2 is proportional to the intensity of this electron current I.
FIG. 2 shows the same cross section through the cathode 2 after it has changed over time. It may be seen that a resistive layer 6, called an interface layer, has grown between the metal support 1 and the washcoat layer 3.
In some applications, it is necessary to try to obtain as high an electron current in the cathode as possible. This is especially the case with cathode-ray tubes for xe2x80x9cmultimediaxe2x80x9d and xe2x80x9chigh-resolutionxe2x80x9d display screens, as well as for video projectors and other types of electron tubes, such as those used in the microwave field.
It is known that the intensity of the electron current that can be obtained from an oxide cathode is limited because it does not have a high enough conductivity. This is essentially the conductivity through the thickness of the washcoat layer 3 and the interface layer 6xe2x80x94that through the support 1 may be regarded as negligible. It should be noted that the conductivity of a layer is inversely proportional to its resistivity.
Moreover, it appears that oxide cathodes do not withstand a high current density well, particularly when the current is constant over time, on account of their insufficient electrical conductivity.
It is generally accepted that the insufficient electrical conductivity of oxide cathodes is due to two parameters: the fact that the emissive washcoat 3 is based on oxides which by nature are poor conductors and the fact that the resistive interface layer 6, between the metal of the support 1 and the washcoat, grows.
FIG. 3 is an equivalent electrical circuit of the components R1 and R2 of the electrical resistivity of the oxide cathode, deriving from the emissive washcoat layer 3 and from the interface layer 6, respectively. As these two layers are superposed, the components R1 and R2 combine as resistors in series.
The contribution of the washcoat layer 3 to the electrical resistivity changes over the lifetime of the cathode. This is because metallic barium is created in this layer by the reaction between the barium oxides BaO and the reducing elements which diffuse out of the nickel. This metallic barium, the primary role of which is to move to the surface of the washcoat in order to allow electron emission, gives the washcoat electrical conductivity. However, the amount of metallic barium decreases for two reasons:
the generation of metallic barium is gradually exhausted because of the fact that the reducing elements must come, by diffusion, from an increasing depth in the nickel; and
the interface layer 6 itself acts as a diffusion barrier with respect to these reducing elements.
The contribution of the interface layer 6 to the electrical conductivity changes during the lifetime because this interface grows. The growth of this interface is due to chemical reactions between the washcoat and the reducing elements contained in the nickel (such as Mg, Si, Al, Zr, W, etc.) which accumulate compounds in this interface. These compounds are rather poor conductors since they are, above all, oxides such as MgO, Al2O3, SiO2, Ba2SiO4, BaZrO3, Ba3WO6, etc.
The origin of the electrical resistivity of oxide cathodes and its changeover time have been studied in the prior art for the purpose of increasing the electron current density that can be sustained.
Certain known solutions aim to reduce the resistivity of the oxide layer 3 by generally incorporating a conductive filler into it. For example:
U.S. Pat. No. 4,369,392 proposes to incorporate nickel powder into the washcoat, which in this case is carried out by pressing and then sintering;
U.S. Pat. No. 4,797,593 provides a solution which comprises adding scandium oxide or yttrium oxide to the washcoat, one of the effects of which is to improve the electrical conductivity;
U.S. Pat. No. 5,592,043 proposes a washcoat in the form of a solid object containing metals (W, Ni, Mg, Re, Mo, Pt) and oxides (of Ba, Ca, Al, Sc, Sr, Th, La) which increase the electrical conductivity by a xe2x80x9cpercolationxe2x80x9d effect; and
U.S. Pat. No. 5,925,976 proposes the addition of metals (Ti, Hf, Ni, Zr, V, Nb, Ta) to the washcoat.
Other known solutions aim to attenuate the effect of the interface layer 6. For example:
U.S. Pat. No. 4,273,683 pertains to the case of an interface formed above all from Ba3WO6. A layer of nickel powder is deposited on the nickel support prior to washcoating and, in addition, a barium carbonate concentration gradient is produced in the thickness of the washcoat. The BaCO3 concentration is less in the region touching the interface, so that less Ba3WO6 compound is created;
U.S. Pat. No. 5,519,280 describes a solution in which indium tin oxide (a complex based on In2O3 and SnO2) is incorporated into the washcoat and acts by providing conductivity and by limiting the growth of the interface;
U.S. Pat. No. 5,977,699 proposes the addition of a zirconium (Zr)-based layer between the nickel of the support and the washcoat, this layer decreasing the interface in terms of its reducing-agent property; and
in the minutes of the conferences xe2x80x9cInternational Vacuum Electron Sources Conferencesxe2x80x9d IVESC98, which were held at Tsukuba (Japan) on Jul. 7-10, 1998, the publication entitled xe2x80x9cAn analysis of the surface of the NiW layer of tungsten film coating cathodexe2x80x9d by Takuya Ohira et al. describes a solution in which a layer of tungsten powder is deposited on the nickel of the support prior to the washcoating and explains that this layer has an effect of dispersing the reducing elements (Si and Mg) so that the compounds (especially Ba2SiO4) resulting from the chemical reactions at the interface are less concentrated and that, consequently, the interface is less of a barrier.
It has also been proposed in U.S. Pat. No. 4,924,137 to ensure that the barium produced by reaction between the oxide layer and the support is absorbed in the washcoat rather than disappearing by evaporation. For this purpose, scandium oxide and an oxide of Al, Si, Ta, V, Cr, Fe, Zr, Nb, Hf, Mo, or W are incorporated into the washcoat.
Finally, solutions have also been proposed in the context of so-called directly heated cathodes. By way of example, U.S. Pat. No. 4,310,777 recommends, in the case of a nickel support having a large amount of tungsten, a small concentration of zirconium in the nickel within a relatively narrow range. Similarly, U.S. Pat. No. 4,313,854 proposes, in the case of a nickel support with a high percentage of refractory metal, interposing a layer of metal (Si, B, Ti, Zr, Hf, V, Nb, Ta, Mo, or w) carbides between the nickel and the washcoat so as to limit in this way the growth of the interface.
It should be noted that the solutions of the prior art do not consider, in a unitary way, the properties associated on the one hand with the oxide layer and on the other hand with the interface layer.
Moreover, other types of cathodes exist, called impregnated cathodes, which allow a sustained regime with a high electron current, even if this current is constant over time. These cathodes comprise a porous metal pill impregnated with an emissive material. However, they are complex and their manufacturing costs exclude them from many applications, especially in cathode-ray tubes intended for the commercial market.
In the light of the foregoing, the subject of the present invention is an oxide cathode comprising a support and an oxide layer on the support. It furthermore includes particles of a conducting material having a first end incorporated in the support and a second end lodged in the oxide layer, so as to constitute conducting bridges passing through an interface layer forming between the support and the oxide layer.
Advantageously, the conducting material of the particles is a carbide of one or more metals, for example:
metals of Group IVB, and preferably at least one metal from: titanium (Ti), zirconium (Zr) and hafnium (Hf);
metals of Group VB, and preferably at least one metal from: vanadium (V), niobium (Nb) and tantalum (Ta);
metals of Group VIB, and preferably at least one metal from: chromium (Cr), molybdenum (Mo) and tungsten (W).
The support may be made of metal, preferably a nickel-based metal.
The invention also relates to an electron tube, for example a cathode-ray tube, comprising an oxide cathode of the aforementioned type. The cathode-ray tube may be intended for so-called xe2x80x9cmultimediaxe2x80x9d television applications.
The invention also relates to a process for manufacturing an oxide cathode in which an oxide layer is deposited on a support, this process comprising the steps consisting in:
furnishing that surface of the support which is intended to receive the oxide layer with particles of conducting material so that the particles have a first end incorporated in the support and a second end which is exposed; and
covering the surface with an oxide layer.
According to a first method of manufacture, the step of furnishing the particles of conducting material consists in spreading the particles out over said surface and in applying a force to the particles in order to encrust the first end of the particles in the support.
According to a second method of manufacture, the step of furnishing the particles of conducting material consists in incorporating the particles in the support and in making the second end of the particles stand out by a surface treatment, for example by means of selective chemical etching treatment.
The particles may be incorporated into the support during the metallurgical production of the latter.
When the support is formed by drawing, the second end of the particles is exposed either before or after the drawing.